Quarter-wave (λ/4) plates and half-wave (λ/2) plates are widely and practically used in the field of antireflection films and liquid crystal displays. Most λ/4 plates and λ/2 plates give λ/4 and λ/2 only at certain wavelengths, respectively.
Japanese Patent Provisional Publication Nos. 5(1993)-27118 and 5(1993)-27119 disclose a phase retarder comprising a birefringent film having high retardation and another birefringent film having low retardation. The birefringent films are laminated so that their optical axes may be perpendicularly crossed. If the difference of the retardation between the two films is λ/4 or λ/2 in the whole visible wavelength region, the phase retarder theoretically serves as a λ/4 or λ/2 plate in the whole visible wavelength region, respectively.
Japanese Patent Provisional Publication No. 10(1998)-68816 discloses a phase retarder comprising laminated two polymer films made of the same material. Each of the films gives λ/2 at a certain wavelength, and thereby the phase retarder gives λ/4 in a wide wavelength region.
Japanese Patent Provisional Publication No. 10(1998)-90521 also discloses a phase retarder comprising laminated two polymer films. This phase retarder also gives λ/4 in a wide wavelength region.
As the polymer films in the above phase retarders, stretched films of synthetic polymers such as polycarbonate are used.